1. Field of the Invention
The present invention relates to prosthetic feet, and more particularly to such feet which lack mechanical joints.
2. Description of the Prior Art
One of the more common prosthetic feet is known as the SACH foot characterized by a rigid keel and bolt block surrounded by a layer of somewhat flexible material. Typically, SACH feet have rigid belting which is fastened to a lower surface of the keel and extends forwardly into the toe region of the foot. Also, the heels of SACH feet are cushioned.
Although SACH feet have some advantages over simple peg legs and other completely rigid feet, they are nevertheless very rigid. Because of this rigidity, SACH feet are virtually incapable of dorsiflexion and plantarflexion, as well as inversion and eversion. Consequently, these feet inhibit walking by a user. Also, these feet lack the ability to conform to uneven surfaces so that they are difficult to use under these conditions. Furthermore, the SACH foot, in the absence of a mechanical ankle joint, does not permit transverse rotation of the foot, that is rotation in a horizontal plane, relative to the long axis of the attached leg. For this reason, users of these feet often have an abnormal gait.
U.S. Pat. Nos. 3,098,239, 3,484,871 and 3,833,941 are illustrative of typical SACH feet.
Solid rubber feet have also been employed, but these typically are very heavy and stiff. The excess weight of such feet increases the energy required of a user during walking. That is, during walking a human foot acts like a pendulum that must be accelerated when the swing of the leg is started and decelerated when the heel hits the ground. The energy required to accelerate and decelerate the leg is related to the center of mass of the leg and increases as this center is shifted distally, such as when weight is added to the foot. As an added drawback, amputees typically have fewer muscles to accomplish acceleration and deceleration of the leg so that excess weight of rubber feet is particularly troublesome. Also, the stiffness of such feet inhibits their motion during walking in much the same manner as SACH feet.
In an attempt to provide greater flexibility, simple inflated feet have been employed. Also, feet combining rigid blocks and sponge and vulcanized rubber inserts, the combinations joined together by rayon reinforcing and binding cord have also been employed. One such foot is known as the Jaipur foot and was developed in India. Although such feet are usually flexible enough to conform to uneven ground, they suffer from the disadvantage of being too flexible during use. This is particularly true during toe-off, when in the course of walking a user's weight is shifted forwardly to the toe region of the foot just prior to picking the foot off the ground. That is, this flexibility inhibits the user's ability to thrust forwardly off the toes.
Still other forms of prior art feet have attempted to provide mobility through the use of mechanical joints. However, such joints tend to fail as a result of the continuously and repetitively applied stresses during use. Furthermore, in many of these types of feet, the joints are exposed where they can trap dirt, grit, and other abrasive and corrosive materials. This contributes to their more rapid failure. Furthermore, feet with mechanical joints are typically more difficult and costly to manufacture. In addition, mechanical joints tend to add weight to the foot which, as explained above, increases the energy required by a user during walking. U.S. Pat. No. 2,556,525 is exemplary of such devices and includes an elongated keel of a relatively stiffened material, for example, vulcanized rubber, surrounded by a soft sponge rubber material with a steel anchor plate embedded in the foot to provide a lower anchor for springs used in an ankle joint. Other examples of such devices are disclosed in U.S. Pat. Nos. 43,031, 961,582, 1,294,632 and 2,453,969.
Therefore, a need exists for a prosthetic foot which, without mechanical joints, is sufficiently rigid and yet is capable of desirable movement during walking.